1. Field of the Invention:
This invention relates to a silver catalyst to be used for the production of ethylene oxide by the catalytic gas-phase oxidation of ethylene with molecular oxygen and to a method for the manufacture of the silver catalyst.
2. Description of the Prior Art:
The silver catalyst which is used in the commercial production of ethylene oxide by the catalytic gas-phase oxidation of ethylene with molecular oxygen is required, for satisfactory performance of the function thereof, to exhibit high selectivity and high activity and enjoy a long catalyst life as.
Various studies have been made to date for the purpose of improving the performance of the silver catalyst and consequently fulfilling the requirement and efforts have been made to improve carriers, reaction promoters, silver compounds and the like. Numerous reports convering published. U.S. Pat. No. 3,207,700 and Japanese Patent Publications SHO 43(1968)-13,137, SHO 45(1970)-21,373, SHO 45(1970)-22,419 and SHO 45(1970)-11,217, Japanese Patent Laid-Open SHO 56(1981)-89,843, U.S. Pat. Nos. 2,766,261, 3,172,893, and 3,664,970 are their examples. Most of them, however, concern pore distributions and specific surface areas of carriers.
In U.S. Pat. No. 2,125,333, there is a description to the effect that an alkali metal salt containing sodium or potassium and a metal salt thereof is used as an additive for the silver catalyst in the manufacture of ethylene oxide.
In U.S. Pat. No. 2,238,474, there is a description to the effect that sodium hydroxide improves the activity of the silver catalyst for the production of ethylene oxide and potassium hydroxide has an adverse effect upon the activity of the silver catalyst.
In U.S. Pat. No. 2,765,283, there is a description to the effect that the silver catalyst is improved by adding 1 to 2,000 ppm by weight of an inorganic chlorinated substance such as sodium chloride to the catalyst carrier before silver is deposited on the carrier.
In U.S. Pat. No. 2,799,687 there is a description to the effect that a halide such as sodium chloride or potassium chloride, used in an amount of 20 to 16,000 ppm, functions as an inhibitor and induces degradation of the activity of the silver catalyst.
In U.S. Pat. No. 4,007,135, there is disclosed a catalyst for the production of alkylene oxide, which catalyst contains copper, gold, zinc, cadmium, mercury, niobium, tantalum, molybdenum, tungsten, vanadium, or preferably chromium, calcium, magnesium, strontium, and/or more preferably barium, and preferably further an alkali metal, in an amount preceding the amount naturally present as an impurity or cement in the carrier and sufficient to manifest the action of a promoter.
In U.S. Pat. No. 4,168,247, there is disclosed a catalyst for the production of alkylene oxide, which catalyst contains silver deposited on a porous heat-resistant carrier possessing a specific surface area in the range of 0.05 to 10 m.sup.2 /g and further contains sodium and at least one other alkali metal selected from the group consisting of potassium, rubidium, and cesium in a promoting amount in excess of the amount naturally present as an impurity or a binding agent in the carrier.
In U.S. Pat. No. 4,278,562, there is disclosure to the effect that a catalyst for the production of an alkylene oxide is obtained by depositing silver and optionally sodium or lithium in the subsequent treatment, depositing thereon the salts of such alkali metals as potassium, rubidium, and cesium in conjunction with an amine and/or ammonia.
In Japanese Patent Laid-Open SHO 55(1980)-145,677, there is disclosed a silver catalyst which, as a catalyst for the reaction of oxidation, has silver and, when necessary, further an alkali metal component or an alkaline earth metal component deposited on a non-acidic carrier containing alumina, silica, and titania in a total amount of not less than 99% by weight, containing metal of the Groups Va, VIa, VIIa. VIII, Ib, and IIb of the Periodic Table of Elements in a total amount of less than 0.1% by weight, and assuming no acid color on exposure to methyl red having a pKa value of +4.8.
In Japanese Patent Laid-Open SHO 56(1981)-105,750, there is disclosed a silver catalyst for the production of ethylene oxide, which silver catalyst is prepared by impregnating a carrier using .alpha.-alumina as a principal component thereof and having a sodium content of not more than 0.07% by weight and a specific surface area in the range of 1 to 5 m.sup.2 /g with an impregnation having 0.001 to 0.05 gram equivalent per kg of complete catalyst, of a complex of an alkali metal with boron, a complex of an alkali metal with molybdenum, and/or a complex of an alkali metal with tungsten contained in a decomposable silver solution formulated to give a deposition ratio of 5 to 25% by weight based on the complete catalyst, and then heating and reducing or thermally decomposing the product of impregnation.
In Japanese Patent Laid-Open SHO 57(1982)-107,241, there is disclosed a silver catalyst for the production of ethylene oxide, which catalyst incorporates therein, besides silver, sodium (Na) as a cationic component and chlorine (Cl) as an anionic component in amounts such that the atomic ratio of Cl/Na will be less than 1.
In U.S. Pat. No. 4,415,476, there is disclosed a silver catalyst for the production of etylene oxide, which silver catalyst contains, besides silver, at least sodium and cesium as cationic components and hlorine as an anionic component.
In U.S. Pat. No. 4,368,144, there is disclosed a silver catalyst for the production of ethylene oxide, which silver catalyst contains metallic silver particles deposited in a ratio of 5 to 25% by weight based on complete catalyst on an .alpha.-alumina carrier having a sodium content of not more than 0.07% by weight and a specific surface area in the range of 0.5 to 5 m.sup.2 /g and 0.001 to 0.05 gram equivalent of at least one alkali metal or alkali metal compound per kg of the complete catalyst and in excess of the amount naturally present in the carrier.
Japanese Patent Laid-Open SHO 63(1988)-116,743 disclosed a silver catalyst for the production of ethylene oxide, containing at least sodium, potassium, rubidium, and/or cesium other than silver as cationic components, among other catalytic components, and using a carrier mainly comprising an .alpha.-alumina, having a surface area in the range of 0.6 to 2 m.sup.2 /g, a water absorption ratio in the range of 20 to 50%, a silica content in the range of 0.5 to 12% by weight, a silica content per surface area (% by weight/m.sup.2 /g) in the range of 0.5 to 12, preferably 1 to 8, and a sodium content in the range of 0.08 to 2% by weight.
Numerous silver catalysts have been proposed as described above. Most of them have their catalytic performance enhanced by incorporating in silver catalysts such amounts of alkalis as falling in restricted ranges. Though these catalysts manifest satisfactory initial catalytic performance, they are deficient in terms of service life.
Concerning the carrier of the silver catalyst to be used for the production of ethylene oxide, many points remain yet to be clarified and many problems still await better solutions. For example, the component substances of the carrier, such physical properties of the carrier as specific surface area, pore diameter, pore distribution, pore volume, porosity, particle diameter, and shape, and the chemical properties owned by the such materials for the carrier as .alpha.-alumina, silicon carbide, silica, and zirconia have room for further improvements aimed at optimization.
An object of this invention, therefore, is to provide a novel silver catalyst for the production of ethylene oxide, which silver catalyst combines such qualities as high selectivity, high activity, and long service life, and a method for the production of the silver catalyst.
Another object of this invention is to provide a novel silver catalyst for the production ethylene oxide, which silver catalyst enjoys improved service life owing to the use of an .alpha.-alumina carrier possessing a specific surface area in the range of 0.75 to 5 m.sup.2 /g and an apparent porosity in the range of 45 to 70%, and a method for the production of the silver catalyst.